New Mass Spectrometric Tools and Their Application to Organic Aerosol Studies

The organic fraction of airborne particulate matter is a complex mixture of compounds with a combined concentration on the order of 1 to 10 micrograms per cubic meter. Thousands of compounds may be present in a sample, with individual concentrations between about 0.1 and 10 nanograms per cubic meter. The complexity and low concentration levels associated with ambient samples carry over into laboratory investigations and present a significant analytical challenge. We have been exploring the use of new methods for off-line and on-line analysis of organic matter in particles to study aerosol reactions. Off-line analysis is performed with electrospray ionization and matrix-assisted laser desorption ionization in combination with exact mass measurements and tandem mass spectrometry. These methods have allowed us to characterize hundreds of high molecular weight compounds formed by secondary polymerization of the primary products of alpha-pinene ozonolysis. Polymerization provides a reasonable explanation of how the volatile primary products are able to partition to the particle phase. On-line measurements are made with an aerosol mass spectrometer that provides "soft" ionization (i.e. very little fragmentation) of organic compounds. Soft ionization is needed to reduce the spectral congestion that is normally observed from complex samples. On-line measurements permit reaction kinetics to be studied. Experiments currently underway that will discussed include the kinetics of acid-catalyzed polymerization in particles and the effect of surface organic layers on the reactive uptake of nitric acid into aqueous sodium chloride particles.